In apparatuses handling confidential information of which risk of leakage is extremely high such as an encryption apparatus for administrating a private key used for encryption, etc., there exists a risk of confidential information stolen by analyzing the operation of CPU (Central Processing Unit) and memories embedded in the apparatus using a logic analyzer, etc. Therefore, measure has been taken to secure safety of the whole apparatus using tamper resistant technique by wrapping the whole apparatus with a metal cover and automatically erasing the confidential information on detecting the fact that the cover is removed.
A conventional tamper resistant apparatus includes a magnetic flux generating device in a cover of the apparatus and a magnetic flux detecting device is connected on the board. Opening the cover changes relative position of the magnetic flux generating device and the magnetic flux detecting device, which enables the magnetic flux detecting device to detect the change of the magnetic flux. Therefore, the tamper resistant apparatus, on detecting the change of the magnetic flux, carries out a tamper resistant process such as erasing the confidential information stored in a volatile memory by suspending the power supply to the volatile memory (Patent Document 1, for example).
Further, there sometimes exist not only one but plural pieces of confidential information. In such a case, another method is proposed to prevent leakage of key information by detecting removal of a cover that wraps the apparatus and erasing all of the plural pieces of confidential information stored in a memory device (Patent Document, for example).    Patent document 1: JP2006-229667A pp. 3-4, FIG. 3    Patent document 2: JP2006-190222A pp. 5-7, FIG. 1